Misfire Under Heavy Lload

I have a 2009 Ariens 27" snow blower with a Briggs & Stratton, model 1150, 250cc OHV engine.

The engine starts and runs perfectly at all times except when it is under really heavy load. Then it will misfire and blow a puff of black smoke out of the exhaust, continue to run okay under load then repeat the misfire a few moments later. It doesn't stall, and stops misfiring as soon as the load eases up even just a little.
I installed a new, properly gapped spark plug and added Sea Foam to the gas, but no improvement. The machine is run dry of fuel before summer storage.

Any ideas? Thanks,
Todd

 

If you can adjust the valves on that engine, do that and if the problem persists, try running a tank of premium gasoline in it. It may be spark knock - predetonation.

 

Does the carburetor drip gas when it's not being used? Sounds to me like it's running a bit rich. Is the air filter wet, dark, or oily?

 

I will try high octane gas.
No dripping from the carb, but suppose the air filter could be iced up from inhaling fine powdered snow. We've had several storms with no temps above freezing for weeks. I'll check.

Thanks for the tips.